It is common knowledge in the papermaking industry that sizing and coating brightness can be significantly improved by the addition of optical brighteners. Optical brighteners also known as OBAs work closely with sizing agents and starch to stay in the coating and sizing matrix. When observed under light, OBAs can absorb invisible ultraviolet light and re-emit visible blue light back, giving viewers the visual perception of a brighter sheet.
Over the past decades, there has been an increasing demand for coated paper of higher brightness. With the addition dosage of OBA growing to satisfy this need, side-effects are observed.
Conventional tetra-sulfonated stilbene optical brightener has a relatively lower cost compared to most others. However only 15 lb/ton maximum addition can be made to the coating/sizing before overdosing occurs which leads to sheet yellowing and over-greening. Even though fluorescence gains will continue to rise, whiteness and brightness will drop.
To combat this effect, further brightness gains has to be obtained from using conventional hexa-sulfonated stilbene OBA which can be added on top for another 20-30 lb/ton before similar effects can be observed. The downside of using this product is the high cost associated with this compound.
In addition, the effect of OBA addition on sizing/coating alone is not optimal. When the hot base sheet is fed through the coater or size press, the optical brightener gets absorbed from sizing/coating medium into the sheet fibres, which prevents proper OBA functions to some extent. It is by the mechanism that the coating/sizing composition has a natural tendency to transfer onto the base sheet all or part of the water and along with the water-soluble materials it contains. Therefore, the characteristic of water retention in the medium is highly sought after.
Often, papermakers utilizes carriers to increase the effectiveness of optical brightener by increasing its retention to stay on the medium and the surface of the sheet. The most commonly used carrier is polyvinyl alcohol (PVOH) with a hydrolysis level of ˜97-99%. The material needs to be precooked fully in order to be in a usable state for production. There is a cost associated with cooking PVOH at the mill, and purchasing pre-cooked PVOH as liquid form is even more expensive. In many cases, the in house cooking cost is also compounded further with inadequate equipment or poorly trained operators.
Papermakers occasionally also choose other natural or synthetic additives to improve the OBA and water retention. Carboxymethyl cellulose (CMC), starch, and other polymers are used. These additives do not degrade paper quality, and can even add improvements to gloss, wet strength, and smoothness.